One of this trait is not seen in a person with Down syndrome

The correct option is High muscle tone.

Down syndrome (Trisomy 21) is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. This condition leads to various physical and developmental characteristics.

Let's analyze the characteristics associated with Down syndrome:
1. Small stature: Individuals with Down syndrome often grow slowly and are typically shorter in height compared to their peers.
2. Upward slant eye: This is a common craniofacial feature, also known as upward-slanting palpebral fissures.
3. Short neck: A short neck, sometimes with excess skin at the back, is another common physical characteristic of this syndrome.
4. Muscle tone: In contrast to high muscle tone, individuals with Down syndrome typically exhibit low muscle tone (hypotonia), which makes them appear floppy and can affect their motor skills and strength.

Therefore, "High muscle tone" is not seen in a person with Down syndrome; instead, low muscle tone is a characteristic feature.